Marking films are recognized in the art as laminates of a self-supporting polymeric film facestock or backing such as polyvinyl chloride (PCV) and a layer of a pressure sensitive adhesive (PSA) for application to substrates, especially vertically oriented substrates. The exposed surface of the PSA of the laminate is in contact with a release liner or the surface of the facestock which is provided with a release coating. Marking films are manufactured in the form of large rolls of the laminate then cut to size depending on the application. Applications may range from automotive pinstripping to large printed signs or individual indicia in the form of or containing letters, numbers, logos and the like.
There are many requirements for a laminate to be used for a marking film. A very important requirement is durability of the laminate of the polymeric film and adhesive for a period of months or years. The laminate of the polymeric film facestock and adhesive must remain functional for an expected period of time and after that period of time, the laminate must have retained the ability to be cleanly removed from the substrate without leaving an adhesive residue. Removal typically occurs with destruction of the facestock namely, where the facestock is no longer reusable. Accordingly, the adhesives are typically regarded more as "permanent" than as "removable".
Polyvinyl chloride films that have been used for marking the film application have varied in durability depending on service life. Short life vinyl films are used for promotional sign applications where an expected service life is six months to one year and if plasticized, the vinyl is typically film plasticized with a monomeric plasticizer. Cost is kept low by elimination of any primer or anchor and barrier layers. Medium and long life films are typically expected to have an useful life of up to about ten years and normally contain UV and heat stabilizing components and polymeric plasticizers. A primer may be employed to enhance anchorage of the pressure-sensitive adhesive to the film and a barrier layer may be employed to inhibit migration of plasticizer from the body of the film into the adhesive. The medium life films are generally calendered and have an expected service life of five years. Long life films are cast and have an expected service life of ten or more years. Vinyl films typically have a thickness of from about 1 to 5 mils, typically 2 to 3 mils.
The PSAs typically used for marking films have been solvent based adhesives. PSAs are typically regarded as "permanent" in the sense that an attempt is made to remove a laminate having a paper facestock from the substrate, tear of the facestock would occur. When used on a vinyl or other polymeric substrates, attempts at removal result in deformation of the facestock to the extent that it may not be reused. The preferred adhesives desirably have some degree of initial repositionability, when the laminate is first applied to the substrate. Repositionability of the laminate without deformation of the facestock ends as adhesion grows with time in consequence of molecular flow to conform to the substrate. Typically the bond to a substrate will increase as much as 100% from an initial peel strength of about 500 N/M after 10 to 15 minutes dwell on the substrate to about 600 to 700 N/M within 24 hours. The requirement that the adhesive provide sufficient bond such that the facestock is not reusable is an important characteristic to inhibit vandalism of applied signs and other indicia in the form of advertisements, labels, and the like.
Other properties must also be present. The PSA must have sufficient shear strength to resist flow under the load of a thick polymeric facestock, it must yield to shrinkage of the facestock without exposing the adhesive, and it must be resistant to adhesive shrinkage so as to avoid wrinkling of the facestock. For outdoor applications, the PSA must be water-resistant with minimal to no significant loss of adhesion. Examples of typical outdoor applications include where the PSA is provided as a marking film label on a container exposed to sea water under transoceanic shipping conditions or as a sign on public or private transportation in climates of high humidity or rainfall. Where the PSA is bonded to a transparent substrate such as glass, it is desirable that the PSA be clear or transparent and resist the whitening action of water.
Another PSA property important to the marking films applications of signs and labels to truck sidings and the like is its compatibility with the practice of spray-washing of the substrate with a surfactant solution, laying the laminate on the clean, wet substrate, and after positioning, squeegeing out the excess water. This presents the problem of water whitening, the resistance to which, as indicated, is important especially where the substrate is transparent. If water whitening occurs it must dissipate in a matter of a few hours.
In consequence of the significant requirements placed on these adhesives, particularly that of resistance to water, the adhesives currently used for marking film applications have been organic solvent-based. Organic solvents are known to be noxious to the senses and may be hazardous. In addition, they present environmental problems, all of which are avoided if the PSA is water-based, such as emulsion-based PSAs. PSAs are, however, notoriously water-sensitive and whiten in some instances by mere application of a drop of water. While some such as a 2-ethylhexyl acrylate-butyl acrylate-vinyl acetate emulsion copolymer, have been employed for general purpose short life marking film applications, there has not yet been proposed functional adhesives for medium and longer term marking film service applications. The present invention is, therefore, directed to water-resistant, i.e., resistant to water-whitening and water deterioration, PSAs where the carrier is water, and which, when employed in marking film label applications, will cleanly remove from the substrate even after many years of exposure to the elements.